Power supply by wiring may be unsuitable for some devices, such as sensors, that are installed at many outdoor locations. Thus, energy harvesting, which is a technique for generating power to be used for operation by use of environmental energy such as sun light, is being considered for application to devices installed at such locations. Energy harvesting is advantageous not only in increasing the flexibility of installation locations for power-using devices, but also in reducing carbon dioxide.
Energy generated by energy harvesting is, however, minute. In addition, with energy harvesting, it is difficult to supply stable energy because the amount of power generated and stored in a battery fluctuates depending on the environment. Since a circuit such as a processor or a sensor operates with stably-supplied power, in order for a processor or the like to operate using power generated by energy harvesting, a regulator such as a DC-DC converter or a low-dropout (LDO) regulator is used to stabilize power supplied to the processor or the like.
However, a regulator uses a diode for overvoltage protection, and the diode allows a leakage current to flow. Thus, a minute amount of power generated by energy harvesting and stored in a battery is consumed by the leakage current through the diode before voltage sufficient to operate a circuit such as a processor is reached. It is therefore difficult to operate the circuit such as a processor with a minute amount of power generated by energy harvesting.
It is conceivable to operate the circuit such as a processor with power generated by energy harvesting and stored in the battery without the intervention of the regulator. However, since the amount of power generated by energy harvesting fluctuates depending on the environment, when a large amount of power is generated, an excessive amount of power is stored in the battery, causing a voltage exceeding the withstand voltage of the circuit such as a processor to be supplied to the circuit such as a processor and hence breaking the circuit such as a processor.
The following is a reference document.    [Document 1] Japanese Laid-open Patent Publication No. 2012-253621.